The invention relates to a method and an apparatus for establishing and/or monitoring the filling level of a medium in a container and to the determination of the density of a medium in a container in accordance with the preamble of claims 1 and 10, respectively.
Apparatuses having at least one oscillating element, so-called vibration detectors, for detecting or for monitoring the filling level of a medium in a container have already become known. The oscillating element is usually at least one oscillating bar which is fastened to a diaphragm. The diaphragm is excited to oscillate via an electromechanical transducer, for example a piezoelectric element. The oscillations of the diaphragm also cause the oscillating element fastened to the diaphragm to execute oscillations.
Vibration detectors constructed as filling level measuring instruments utilize the effect that the oscillation frequency and the oscillation amplitude depend on the respective degree of coverage of the oscillating element: whereas the oscillating element can execute its oscillations freely and undamped in air, it experiences a change in frequency and amplitude as soon as it is immersed partially or completely in the medium. Consequently, a predetermined change in frequency (the frequency is usually measured) can be used to draw an unambiguous conclusion on the achievement of the predetermined filling level of the medium in the container. Furthermore, filling level measuring instruments are chiefly used as protection against overfilling or for the purpose of safeguarding against pumps running dry.
Moreover, the damping of the oscillation of the oscillating element is also influenced by the respective density of the medium. Consequently, in the case of a constant degree of coverage there is a functional relationship with the density of the medium, with the result that vibration detectors are better suited both for determining filling level and for determining density. In practice, for the purpose of monitoring and detecting the filling level or the density of the medium in the container the oscillations of the diaphragm are picked up and converted into electrical reception signals with the aid of at least one piezoelectric element.
The electrical reception signals are subsequently evaluated by an electronic evaluation system. In the case of determining the filling level, the electronic evaluation system monitors the frequency of oscillation and/or the amplitude of oscillation of the oscillating element and signals the state of xe2x80x9csensor coveredxe2x80x9d or xe2x80x9csensor uncoveredxe2x80x9d as soon as the measured values undershoot or overshoot a prescribed reference value. An appropriate message to the operating staff can be made optically and/or acoustically. A switching operation is triggered as an alternative or in addition; for example, a feed valve or discharge valve on the container is opened or closed.
The instruments previously mentioned for measuring the filling level or the density are used in many sectors of industry, for example in the chemical industry, in the foodstuffs industry or in water treatment. The bandwidth of the monitored charge materials ranges from water through yoghurt, colorants and coatings to highly viscous charge materials such as honey or up to greatly foaming charge materials such as beer.
The measurement of filling level or density by means of vibration detectors encounters problems due to the fact that there is no unique relationship between a change in frequency which occurs and the degree of coverage or the density of the medium. An important influencing quantity which makes itself felt just like the coupling to the mass of the medium in a shift in the resonant frequency is the change in mass of the oscillable unit. A change in mass can be caused both by the formation of coating, that is to say by the formation of deposits of the medium on the oscillable unit, and by the corrosion of the oscillating bars. Depending on the type of change in mass and its degree, it is possible here for the most highly undesired case to occur, that the sensor is permanently xe2x80x9ccoveredxe2x80x9d or xe2x80x9cuncoveredxe2x80x9d, and thus that it is reported that the predetermined filling level has been reached although the limit filling level has not yet been reached. A similar statement holds with regard to the measurement of density: the false density of the medium is measured and indicated.
It is the object of the invention to propose a method and an apparatus which permit reliable determination and/or monitoring of the filling level or the density of a medium.
The object is achieved with regard to the method by virtue of the fact that at least a first mode and a second mode of the oscillations of the oscillable unit are evaluated, and that the evaluated modes are used to detect a change in mass of the oscillable unit. Although exclusive reference is frequently made below to an increment in mass which occurs in the event of the formation of coating on the oscillable unit, comparable considerations hold, of course, for the loss of mass which can be, inter alia, a consequence of the corrosion of the oscillating bars.
The invention is based on the physical effect that different oscillation modes are formed upon excitation of the oscillable unit. It is set forth below in yet more detail which different oscillation modes occur for a vibration detector having, for example, paddle-shaped oscillating bars.
In accordance with an advantageous development of the method according to the invention, it is provided that modes whose oscillations are differently influenced by the medium are evaluated as first and second mode.
A preferred variant of the method according to the invention proposes that the first mode is a mode whose oscillations are essentially independent of the medium, and that the second mode is a mode whose oscillations are influenced by the medium. In concrete terms, this means that the first mode is selected as a mode whose natural frequency or resonant frequency is shifted as a consequence of a change in mass, but whose resonant frequency remains essentially unchanged when the oscillable unit comes into contact with the medium. Consequently, the first mode can be any mode in which the cross-sectional surfaces of the xe2x80x9coscillating bars/mediumxe2x80x9d of the oscillating bars are small in the direction of oscillation. If this precondition is fulfilled, the interaction of the oscillable unit with the medium, and thus the mass coupling of the oscillable unit to the medium is relatively weak. Selected as second mode is a mode whose natural frequency changes strikingly as soon as the oscillable unit comes into contact with the medium.
An advantageous development of the method according to the invention provides that a change in the first mode whose oscillations are essentially independent of the medium is used to detect whether a change in mass has occurred at the oscillable unit. It is provided, in particular, that a change in frequency of the oscillations of the first mode is used to detect a formation of coating or a loss in mass at the oscillable unit.
Whereas the previously described first variant of the method according to the invention provides for the selection of two modes which exhibit entirely different reactions as a consequence of the change in mass or as a consequence of the contact with the medium, a second variant adopts a different avenue. In accordance with the alternative second variant of the method according to the invention, it is provided to select two modes as first mode and as second mode of the oscillations of the oscillable unit, the two modes respectively having a first component which is a function of the coupling to the mass of the medium, and the two modes having a second component which is independent of the coupling to the mass of the medium and which depends only on the respective mass of the oscillable unit.
An advantageous development of the method according to the invention provides for drawing conclusions on the change in mass of the oscillable unit with the aid of the functional relationship of the first and the second modes of the oscillations of the oscillable unit on the medium and on the mass of the oscillable unit. The sole requirement which is to be made of the selection of the two modes is that they differ sufficiently from one another.
The determination of the influence of the formation of coating on the measured values is preferably performed via a system of equations which is composed of the two formulas named below:
xe2x80x83xcex94FC=ƒC1(mk)+ƒC2(ma)
xcex94FD=ƒD1(mk)+ƒD2(ma)
The symbols used in this system of equations characterize the following variables:
xcex94Fc: the relative frequency shift of a first mode;
xcex94FD: the relative frequency shift of a second mode;       Δ    ⁢          xe2x80x83        ⁢          F      ⁡              [        %        ]              =            (                                                  F              measure                        ⁡                          [              Hz              ]                                                          F                              air                ,                without_coating                                      ⁡                          [              Hz              ]                                      -        1            )        *    100    ⁢    %  
xe2x80x83the term respectively symbolizing the relative frequency shift in the natural frequency of the corresponding mode, relative meaning that the measured frequency shift is expressed in percent with reference to the corresponding natural frequency in air without the formation of coating.
mk: a measure of each type of mass coupling to and damping by the medium. Herexe2x80x94as already described earlierxe2x80x94in addition to the depth of immersion h of the oscillable unit, a role is also played by the density xcfx81 of the medium and the viscosity xcex7 of the medium. In computational terms this can be expressed by the following functional relationship: mk=f(h;xcfx81,xcex7);
ma: the coating mass;
ƒc1(mk), ƒD2(mk): the frequency shift curves of two sufficiently different modes (for example, mode C and mode D) of the oscillable unit, as a function of the mass coupling mk of the oscillable unit, and the damping of the oscillable unit by the medium (xe2x86x92 immersion curves);
ƒc2(ma), ƒD2(ma): the frequency shift curves of two sufficiently different modes (for example, mode C and mode D) of the oscillable unit as a function of the formation of coating ma on the oscillable unit (xe2x86x92 coating curves).
An advantageous refinement of the method according to the invention provides that an error message is output when the changes in frequency, caused by change in mass of the oscillable unit, of a first and/or a second mode of the oscillations of the oscillable unit overshoot a prescribed desired value.
It is particularly advantageous when a change, caused by change in mass at the oscillable unit, in a first and/or a second mode of the oscillations of the oscillable unit is used for the purpose of undertaking inline correction of the measured data of the oscillable unit.
With reference to the apparatus according to the invention, the object is achieved by virtue of the fact that the control/evaluation unit uses at least a first mode and a second mode of the oscillations of the oscillable unit for the purpose of evaluation, and that the control/evaluation unit detects a change in mass at the oscillable unit with the aid of the evaluated modes.
An advantageous refinement of the apparatus according to the invention provides that the evaluation/control unit is integrated into the apparatus for determining and/or monitoring the filling level and/or for determining the density of the medium. The apparatus according to the invention is in this case a so-called compact sensor. The error message can be digitally output, for example optically, acoustically and/or via at least two data lines.
A refinement of the apparatus according to the invention which is alternative to the compact sensor provides at least two data lines via which the measured data are led to the evaluation/control unit or via which the evaluation/control unit communicates with a remote control point. It is particularly advantageous in this connection when the respective measured data and/or correction data are transmitted digitally to the remote control point. By contrast with analog data transmission, digital data communication has the known advantage of increased interference immunity. Of course, recourse may be made to the known transmission protocols and transmission standards for the communication.
In accordance with a preferred development of the apparatus according to the invention, an output unit is proposed which outputs an error message to the operating staff optically and/or acoustically when, preferably within the limits of prescribed tolerance values, a prescribed desired value of the change in frequency which is to be ascribed to a change in mass of the oscillable unit is overshot or undershot.
Moreover, it is advantageously provided that the control/evaluation unit is assigned a storage unit in which desired values are stored for tolerable changes in frequency which originate from a change in mass.